Magnetic tape and magnetic recording and reproducing device

ABSTRACT

Provided are a magnetic tape comprising a magnetic layer containing a ferromagnetic powder and a binding agent on a non-magnetic support, in which the magnetic layer contains an oxide abrasive, an average particle diameter of the oxide abrasive obtained from a secondary ion image acquired by irradiating a surface of the magnetic layer with a focused ion beam is greater than 0.08 μm and 0.14 μm or smaller, and an absolute value ΔN of a difference between a refractive index Nxy measured with respect to an in-plane direction of the magnetic layer and a refractive index Nz measured with respect to a thickness direction of the magnetic layer is 0.25 to 0.40, and a magnetic recording and reproducing device including the magnetic tape.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT International Application No.PCT/JP2018/033531 filed on Sep. 11, 2018, which claims priority under 35U.S.C. § 119(a) to Japanese Patent Application No. 2017-191660 filed onSep. 29, 2017 and Japanese Patent Application No. 2018-159515 filed onAug. 28, 2018. Each of the above applications is hereby expresslyincorporated by reference, in its entirety, into the presentapplication.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape and a magneticrecording and reproducing device.

2. Description of the Related Art

Magnetic recording media are divided into tape-shaped magnetic recordingmedia and disk-shaped magnetic recording media, and tape-shaped magneticrecording media, that is, magnetic tapes are mainly used for datastorage. The recording of information on a magnetic tape and/orreproducing thereof are normally performed by causing a surface of themagnetic tape (a surface of a magnetic layer) to come into contact witha magnetic head (hereinafter, also simply referred to as a “head”) forsliding. As the magnetic tape, a magnetic tape having a configuration inwhich a magnetic layer containing a ferromagnetic powder and a bindingagent is provided on a non-magnetic support is widely used (for example,see JP2005-243162A).

SUMMARY OF THE INVENTION

In a case of reproducing information recorded on a magnetic tape, as afrequency of generation of a partial decrease in reproducing signalamplitude (referred to as a “missing pulse”) increases, an error rateincreases and reliability of the magnetic tape decreases. Therefore, inorder to provide a magnetic tape capable of being used with highreliability, it is desired to decrease a generation frequency of themissing pulse.

In recent years, magnetic tapes used for data storage may be used in alow temperature and low humidity environment such as data centers wheretemperature and humidity are controlled (for example, in an environmentwhere the temperature is 10 to 15° C. and the relative humidity is about10 to 20%). Therefore, it is desirable to reduce the generationfrequency of the missing pulses in a low temperature and low humidityenvironment.

An object of the invention is to provide a magnetic tape in which ageneration frequency of a missing pulse in the low temperature and lowhumidity environment is decreased.

One aspect of the invention relates to a magnetic tape comprising amagnetic layer containing a ferromagnetic powder and a binding agent ona non-magnetic support, in which the magnetic layer contains an oxideabrasive, an average particle diameter (hereinafter, also referred to asa “FIB abrasive diameter”) of the oxide abrasive obtained from asecondary ion image acquired by irradiating a surface of the magneticlayer with a focused ion beam (FIB) is greater than 0.08 μm and 0.14 μmor smaller, and an absolute value ΔN (hereinafter, also referred to as“ΔN (of the magnetic layer)”) of a difference between a refractive indexNxy measured with respect to an in-plane direction of the magnetic layerand a refractive index Nz measured with respect to a thickness directionof the magnetic layer is 0.25 to 0.40.

In one aspect, the oxide abrasive may be an alumina powder.

In one aspect, the difference (Nxy−Nz) between the refractive index Nxyand the refractive index Nz may be 0.25 to 0.40.

In one aspect, the ferromagnetic powder may be a ferromagnetic hexagonalferrite powder.

In one aspect, the magnetic tape may further comprise a non-magneticlayer containing a non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.

In one aspect, the magnetic tape may further comprise a back coatinglayer containing a non-magnetic powder and a binding agent on a surfaceof the non-magnetic support opposite to a surface provided with themagnetic layer.

Another aspect of the invention relates to a magnetic recording andreproducing device comprising the magnetic tape, and a magnetic head.

According to one aspect of the invention, it is possible to provide amagnetic tape in which a generation frequency of a missing pulse in thelow temperature and low humidity environment is decreased. In addition,according to another aspect of the invention, it is possible to providea magnetic recording and reproducing device including the magnetic tape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Magnetic Tape]

One aspect of the invention relates to a magnetic tape comprising amagnetic layer containing a ferromagnetic powder and a binding agent ona non-magnetic support, in which the magnetic layer contains an oxideabrasive, an average particle diameter (a FIB abrasive diameter) of theoxide abrasive obtained from a secondary ion image acquired byirradiating a surface of the magnetic layer with a focused ion beam isgreater than 0.08 μm and 0.14 μm or smaller, and an absolute value ΔN ofa difference between a refractive index Nxy measured with respect to anin-plane direction of the magnetic layer and a refractive index Nzmeasured with respect to a thickness direction of the magnetic layer is0.25 to 0.40.

In the invention and the specification, the “surface of the magneticlayer” is identical to a surface of the magnetic tape on the magneticlayer side. In the invention and the specification, the “ferromagneticpowder” means an aggregate of a plurality of ferromagnetic particles.The “aggregate” is not only limited to an aspect in which particlesconfiguring the aggregate directly come into contact with one another,but also includes an aspect in which a binding agent, an additive, orthe like is interposed between the particles. The points described aboveare also applied to various powders such as the non-magnetic powder ofthe invention and the specification, in the same manner.

In the invention and the specification, the “oxide abrasive” means anon-magnetic oxide powder having a Mohs hardness of greater than 8.

In the invention and the specification, the FIB abrasive diameter is avalue obtained by the following method.

(1) Acquisition of Secondary Ion Image

A secondary ion image of a 25 μm square (25 μm×25 μm) region on thesurface of the magnetic layer of the magnetic tape for which FIBabrasive diameter is to be obtained is acquired by a focused ion beamdevice. As a focused ion beam device, MI4050 manufactured by HitachiHigh-Technologies Corporation can be used.

The beam irradiation conditions of the focused ion beam device foracquiring the secondary ion image are set to an acceleration voltage of30 kV, a current value of 133 pA (picoampere), a beam size of 30 nm, anda brightness of 50%. The coating process before imaging on the magneticlayer surface is not performed. A secondary ion (SI) signal is detectedby the secondary ion detector, and a secondary ion image is captured.The imaging conditions for the secondary ion image are determined by thefollowing method. By performing auto contrast brightness (ACB) (that is,performing ACB three times) at three unimaged regions on the surface ofthe magnetic layer, the tint of the image is stabilized and a contrastreference value and a brightness reference value are determined. Thecontrast value obtained by lowering the contrast reference valuedetermined by the ACB by 1% and the above brightness reference value areset as the imaging conditions. An unimaged region on the surface of themagnetic layer is selected, and the secondary ion image is capturedunder the imaging conditions determined above. A portion for displayingthe size or the like (micron bar or cross mark) is deleted from thecaptured image, and the secondary ion image having the number of pixelsof 2000 pixels×2000 pixels is acquired. For specific examples of theimaging conditions, the examples described below can be referred to.

(2) Calculation of FIB Abrasive Diameter

The secondary ion image acquired in the section (1) is taken into imageprocessing software and binarized by the following procedure. As theimage analysis software, for example, ImageJ which is free software canbe used.

The color tone of the secondary ion image acquired in the section (1) ischanged to 8 bits. The threshold values for binarization processing area lower limit value of 250 gradation and an upper limit value of 255gradation, and the binarization processing is executed with these twothreshold values. After the binarization processing, noise componentremoval processing is performed by image analysis software. The noisecomponent removal processing can be performed by the following method,for example. In the image analysis software ImageJ, the noise cutprocessing Despeckle is selected, and size 4.0-infinity is set inAnalyze Particle to remove the noise component.

In the obtained binarized image, each portion that shines white isdetermined as the oxide abrasive, the number of portions that shinewhite is obtained by image analysis software, and the area of eachportion that shines white is obtained. The equivalent circle diameter ofeach portion is obtained from the obtained area of each portion shiningwhite. Specifically, the equivalent circle diameter L is calculated fromthe obtained area A by (A/π){circumflex over ( )}(½)×2=L.

The above process is performed four times at different locations (25 μmsquare) on the surface of the magnetic layer of the magnetic tape forwhich the FIB abrasive diameter is to be obtained, and from the obtainedresults, the FIB abrasive diameter is calculated by FIB abrasivediameter=Σ(Li)/Σi. Σi is the total number of portions that shine whiteobserved in the binarized image obtained by performing the process fourtimes. Σ(Li) is the sum of equivalent circle diameters L obtained foreach portion that shines white observed in the binarized image obtainedby performing the process four times. Only a part of the part whichshines white may be included in the binarized image. In such a case, Σiand Σ(Li) are obtained without including that portion.

In the invention and the specification, the absolute value ΔN of thedifference between the refractive index Nxy measured regarding thein-plane direction of the magnetic layer and the refractive index Nzmeasured regarding the thickness direction of the magnetic layer is avalue obtained by the following method.

The refractive index regarding each direction of the magnetic layer isobtained using a double-layer model by spectral ellipsometry. In orderto obtain the refractive index of the magnetic layer using thedouble-layer model by spectral ellipsometry, the value of the refractiveindex of a portion adjacent to the magnetic layer is used. Hereinafter,an example in a case of obtaining the refractive indexes Nxy and Nz ofthe magnetic layer of the magnetic tape including a layer configurationin which the non-magnetic layer and the magnetic layer are laminated onthe non-magnetic support in this order will be described. However, themagnetic tape according to one aspect of the invention may also be amagnetic tape having a layer configuration in which the magnetic layeris directly laminated on the non-magnetic support without thenon-magnetic layer interposed therebetween. Regarding the magnetic tapehaving such a configuration, the refractive index regarding eachdirection of the magnetic layer is obtained in the same manner as thefollowing method, using the double-layer model of the magnetic layer andthe non-magnetic support. In addition, an incidence angle shown below isan incidence angle in a case where the incidence angle is 0° in a caseof vertical incidence.

(1) Preparation of Sample for Measurement

Regarding the magnetic tape including a back coating layer on a surfaceof a non-magnetic support on a side opposite to the surface providedwith a magnetic layer, the measurement is performed after removing theback coating layer of a sample for measurement cut from the magnetictape. The removal of the back coating layer can be performed by awell-known method of dissolving of the back coating layer using asolvent or the like. As the solvent, for example, methyl ethyl ketonecan be used. However, any solvent which can remove the back coatinglayer need only be used. The surface of the non-magnetic support afterremoving the back coating layer is roughened by a well-known method sothat the reflected light on this surface is not detected, in themeasurement of ellipsometer. The roughening can be performed by a methodof polishing the surface of the non-magnetic support after removing theback coating layer by using sand paper, for example. Regarding thesample for measurement cut out from the magnetic tape not including theback coating layer, the surface of the non-magnetic support on a sideopposite to the surface provided with the magnetic layer is roughened.

In addition, in order to measure the refractive index of thenon-magnetic layer described below, the magnetic layer is furtherremoved and the surface of the non-magnetic layer is exposed. In orderto measure the refractive index of the non-magnetic support describedbelow, the non-magnetic layer is also further removed and the surface ofthe non-magnetic support on the magnetic layer side is exposed. Theremoval of each layer can be performed by a well-known method so asdescribed regarding the removal of the back coating layer. Alongitudinal direction described below is a direction which was alongitudinal direction of the magnetic tape, in a case where the samplefor measurement is included in the magnetic tape before being cut out.This point applies to other directions described below, in the samemanner.

(2) Measurement of Refractive Index of Magnetic Layer

By setting the incidence angles as 65°, 70°, and 75°, and irradiatingthe surface of the magnetic layer in the longitudinal direction with anincidence ray having a beam diameter of 300 μm by using theellipsometer, Δ (a phase difference of s-polarized light and p-polarizedlight) and Ψ (an amplitude ratio of s-polarized light and p-polarizedlight) is measured. The measurement is performed by changing awavelength of the incidence ray by 1.5 nm in a range of 400 to 700 nm,and a measurement value at each wavelength is obtained.

The refractive index of the magnetic layer at each wavelength isobtained with a double-layer model as described below, by using themeasurement values of Δ and Ψ of the magnetic layer at each wavelength,the refractive index of the non-magnetic layer in each directionobtained by the following method, and the thickness of the magneticlayer.

The zeroth layer which is a substrate of the double-layer model is setas a non-magnetic layer and the first layer thereof is set as a magneticlayer. The double-layer model is created by assuming that there is noeffect of rear surface reflection of the non-magnetic layer, by onlyconsidering the reflection of the interfaces of air/magnetic layer andmagnetic layer/non-magnetic layer. A refractive index of the first layerwhich is fit to the obtained measurement value the most is obtained byfitting performed by a least squares method. The refractive index Nx ofthe magnetic layer in the longitudinal direction and a refractive indexNz₁ of the magnetic layer in the thickness direction measured byemitting the incidence ray in the longitudinal direction are obtained asvalues at the wavelength of 600 nm obtained from the results of thefitting.

In the same manner as described above, except that the direction ofincidence of the incidence ray is set as a width direction of themagnetic tape, a refractive index Ny of the magnetic layer in the widthdirection and a refractive index Nz₂ of the magnetic layer in thethickness direction measured by emitting the incidence ray in the widthdirection are obtained as values at the wavelength of 600 nm obtainedfrom the results of the fitting.

The fitting is performed by the following method.

In general, “complex refractive index n=η+iκ” is satisfied. Here, η is areal number of the refractive index, κ is an extinction coefficient, andi is an imaginary number. In a case where a complex dielectric constantε=ε1+ε2 (ε1 and ε2 satisfy Kramers-Kronig relation), ε1=η²−κ², andε²=2ηκ, the complex dielectric constant of Nx satisfies thatε_(x)=ε_(x)1+iε_(x)2, and the complex dielectric constant of Nz₁satisfies that ε_(z1)=ε_(z1)1+i_(ε1)2, in a case of calculating the Nxand Nz₁.

The Nx is obtained by setting ε_(x)2 as one Gaussian, setting any point,where a peak position is 5.8 to 5.1 eV and σ is 4 to 3.5 eV, as astarting point, setting a parameter to be offset to a dielectricconstant beyond a measurement wavelength range (400 to 700 nm), andperforming least squares fitting with respect to the measurement value.In the same manner, Nz₁ is obtained by setting any point of ε_(x1)2,where a peak position is 3.2 to 2.9 eV and σ is 1.5 to 1.2 eV, as astarting point, and setting an offset parameter, and performing leastsquares fitting with respect to the measurement value. Ny and Nz₂ arealso obtained in the same manner. The refractive index Nxy measuredregarding the in-plane direction of the magnetic layer is obtained as“Nxy=(Nx+Ny)/2”. The refractive index Nz measured regarding thethickness direction of the magnetic layer is obtained as“Nz=(Nz₁+Nz₂)/2”. From the obtained Nxy and Nz, the absolute value ΔN ofdifference thereof is obtained.

(3) Measurement of Refractive Index of Non-Magnetic Layer

Refractive indexes of the non-magnetic layer at a wavelength of 600 nm(the refractive index in the longitudinal direction, the refractiveindex in the width direction, the refractive index in the thicknessdirection measured by emitting the incidence ray in the longitudinaldirection, and the refractive index in the thickness direction measuredby emitting the incidence ray in the width direction) are obtained inthe same manner as in the method described above, except the followingpoints.

The wavelength of the incidence ray is changed by 1.5 nm in the range of250 to 700 nm.

By using a double-layer model of a non-magnetic layer and a non-magneticsupport, the zeroth layer which is a substrate of the double-layer modelis set as the non-magnetic support, and the first layer thereof is setas the non-magnetic layer.

The double-layer model is created by assuming that there is no effect ofrear surface reflection of the non-magnetic support, by only consideringthe reflection of the interfaces of air/non-magnetic layer andnon-magnetic layer/non-magnetic support.

In the fitting, seven peaks (0.6 eV, 2.3 eV, 2.9 eV, 3.6 eV, 4.6 eV, 5.0eV, and 6.0 eV) are assumed in the imaginary part (82) of the complexdielectric constant, and the parameter to be offset is set to thedielectric constant beyond the measurement wavelength range (250 to 700nm).

(4) Measurement of Refractive Index of Non-Magnetic Support

The refractive indexes of the non-magnetic support at a wavelength of600 nm (refractive index in the longitudinal direction, the refractiveindex in the width direction, the refractive index in the thicknessdirection measured by emitting the incidence ray in the longitudinaldirection, and the refractive index in the thickness direction measuredby emitting the incidence ray in the width direction) used for obtainingthe refractive indexes of the non-magnetic layer by the double-layermodel are obtained in the same manner as in the method described abovefor measuring the refractive index of the magnetic layer, except thefollowing points.

A single-layer model with only front surface reflection is used, withoutusing the double-layer model.

The fitting is performed by the Cauchy model (n=A+B/λ², n is arefractive index, A and B are respectively constants determined byfitting, and λ is a wavelength).

The inventors have surmised as follows regarding a reason for a decreasein the generation frequency of the missing pulse in the low temperatureand low humidity environment in the magnetic tape.

The FIB abrasive diameter is a value that can be used as an index of thepresence state of the oxide abrasive in the magnetic layer and isobtained from the secondary ion image obtained by irradiating thesurface of the magnetic layer with a focused ion beam (FIB). Thesecondary ion image is generated by capturing the secondary ionsgenerated from the surface of the magnetic layer irradiated with theFIB. On the other hand, as a method for observing the presence state ofthe abrasive in the magnetic layer, in the related art, as disclosed inparagraph 0109 of JP2005-243162A, for example, a method using a scanningelectron microscope (SEM) has been proposed. In the SEM, the surface ofthe magnetic layer is irradiated with an electron beam, secondaryelectrons emitted from the surface of the magnetic layer are captured,and an image (the SEM image) is generated. Due to the difference inimage generation principle, even in a case where the same magnetic layeris observed, the size of the oxide abrasive obtained from the secondaryion image is different from the size of the oxide abrasive obtained fromthe SEM image. As a result of intensive studies, the inventors havesurmised that the presence state of the oxide abrasive in the magneticlayer is controlled so that the FIB abrasive diameter is greater than0.08 μm and 0.14 μm or smaller using the FIB abrasive diameter obtainedby the above method from the secondary ion image as a new index of thepresence state of the oxide abrasive in the magnetic layer. Theinventors have considered that the control of the presence state of theoxide abrasive in the magnetic layer contributes to reducing of thegeneration frequency of the missing pulse in the low temperature and lowhumidity environment. The specific description is as follows.

In a case of reproducing information recorded on the magnetic tape, in acase where the surface of the magnetic layer is chipped in the slidingof the surface of the magnetic layer and a head, the generated scrapsare attached to the head and a head attached material may be generated.The head attached material generated in this way can be removed bygiving the surface of the magnetic layer a head cleaning property. As ameans for giving the surface of the magnetic layer the head cleaningproperties, in the related art, for example, as disclosed inJP2005-243162A, a method of containing the abrasive in the magneticlayer has been performed. On the other hand, as the head cleaningproperty of the surface of the magnetic layer is higher, the head iseasily chipped by sliding with the surface of the magnetic layer. Theinventors have found that the cause of generation of the missing pulseis that the contact state in a case of sliding the head with the surfaceof the magnetic layer is unstable, and the cause of unstable contactstate is the generation of the head attached material and the chippingof the head.

Regarding the above points, the inventors have found that the FIBabrasive diameter of 0.14 μm or smaller in the magnetic tape contributesto suppression of the head chipping in the low temperature and lowhumidity environment and the FIB abrasive diameter of greater than 0.08μm contributes to suppression of the head chipping in the lowtemperature and low humidity environment and removing the head attachedmaterial by applying the head cleaning properties on the surface of themagnetic layer.

Furthermore, the inventors have thought that ΔN obtained by the methoddescribed above is a value which may be an index of a presence state ofthe ferromagnetic powder in a surface region of the magnetic layer. Itis assumed that ΔN is a value affected by various factors such as thepresence state of the binding agent or the density distribution of theferromagnetic powder in addition to alignment state of the ferromagneticpowder in the magnetic layer, and the magnetic layer in which each ofthe factors is controlled and ΔN is 0.25 to 0.40 is not easily chippedby sliding the head with the surface of the magnetic layer due to thehigh strength thereof. The inventors have surmised that, thiscontributes to suppression of the generation of the head attachedmaterial due to the chipping of the surface of the magnetic layer in themagnetic layer in which the FIB abrasive diameter is within the aboverange, and as a result, this contributes to a decrease in the generationfrequency of the missing pulse in the low temperature and low humidityenvironment.

However, the above descriptions are merely a surmise of the inventorsand the invention is not limited thereto.

Hereinafter, the magnetic tape will be described more specifically.Hereinafter, the generation frequency of the missing pulse in the lowtemperature and low humidity environment is also simply referred to asthe “generation frequency of the missing pulse”.

<FIB Abrasive Diameter>

The FIB abrasive diameter obtained from the secondary ion image acquiredby irradiating the surface of the magnetic layer of the magnetic tapewith the FIB is greater than 0.08 μm and 0.14 μm or smaller. It isconsidered that the FIB abrasive diameter of 0.14 μm or smallercontributes to suppression of the head chipping by the sliding of thehead with the surface of the magnetic layer in the low temperature andlow humidity environment. Also, it is surmised that the FIB abrasivediameter greater than 0.08 μm contributes to removing of the headattached material generated by chipping the surface of the magneticlayer by the exhibition of the head cleaning properties of the surfaceof the magnetic layer in the low temperature and low humidityenvironment. From a viewpoint of further decreasing the generationfrequency of the missing pulse, the FIB abrasive diameter is preferably0.09 μm or greater, and more preferably 0.10 μm or greater. From thesame viewpoint, the FIB abrasive diameter is preferably 0.13 μm orsmaller and more preferably 0.12 μm or smaller. A specific aspect ofmeans for adjusting FIB abrasive diameter will be described below.

<ΔN of Magnetic Layer>

ΔN of the magnetic layer of the magnetic tape is 0.25 to 0.40. Asdescribed above, it is surmised that the magnetic layer having ΔN of0.25 to 0.40 has a high strength of the surface of the magnetic layer,and the chipping thereof due to the sliding with the head hardly occurs.Accordingly, it is thought that, in a case of reproducing informationrecorded on the magnetic layer in the low temperature and low humidityenvironment, the chipping of the magnetic layer having ΔN in the rangedescribed above hardly occurs on the surface of the magnetic layerduring the sliding of the surface of the magnetic layer and the head. Itis surmised that this contributes to a decrease in the generationfrequency of the missing pulse. From a viewpoint of further decreasingthe generation frequency of the missing pulse, ΔN is preferably 0.25 to0.35. A specific aspect of means for adjusting ΔN will be describedlater.

ΔN is an absolute value of a difference between Nxy and Nz. Nxy is arefractive index measured regarding the in-plane direction of themagnetic layer and Nz is a refractive index measured regarding thethickness direction of the magnetic layer. In one aspect, a relation ofNxy>Nz can be satisfied, and in the other aspect, Nxy<Nz can besatisfied. From a viewpoint of electromagnetic conversioncharacteristics of the magnetic tape, a relationship of Nxy>Nz ispreferable, and therefore, the difference between the Nxy and Nz(Nxy−Nz) is preferably 0.25 to 0.40.

Hereinafter, the magnetic tape will be described more specifically.

<Magnetic Layer>

(Ferromagnetic Powder)

As the ferromagnetic powder contained in the magnetic layer,ferromagnetic powder normally used in the magnetic layer of variousmagnetic recording media can be used. It is preferable to useferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density of the magnetic recordingmedium. From this viewpoint, ferromagnetic powder having an averageparticle size of 50 nm or smaller is preferably used as theferromagnetic powder. Meanwhile, the average particle size of theferromagnetic powder is preferably 10 nm or greater, from a viewpoint ofstability of magnetization.

As a preferred specific example of the ferromagnetic powder,ferromagnetic hexagonal ferrite powder can be used. An average particlesize of the ferromagnetic hexagonal ferrite powder is preferably 10 nmto 50 nm and more preferably 20 nm to 50 nm, from a viewpoint ofimprovement of recording density and stability of magnetization. Fordetails of the ferromagnetic hexagonal ferrite powder, descriptionsdisclosed in paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134to 0136 of JP2011-216149A, and paragraphs 0013 to 0030 of JP2012-204726Acan be referred to, for example.

As a preferred specific example of the ferromagnetic powder,ferromagnetic metal powder can also be used. An average particle size ofthe ferromagnetic metal powder is preferably 10 nm to 50 nm and morepreferably 20 nm to 50 nm, from a viewpoint of improvement of recordingdensity and stability of magnetization. For details of the ferromagneticmetal powder, descriptions disclosed in paragraphs 0137 to 0141 ofJP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A can bereferred to, for example.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on photographicprinting paper so that the total magnification ratio of 500,000 toobtain an image of particles configuring the powder. A target particleis selected from the obtained image of particles, an outline of theparticle is traced with a digitizer, and a size of the particle (primaryparticle) is measured. The primary particle is an independent particlewhich is not aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. A value regarding asize of powder such as the average particle size shown in examples whichwill be described later is a value measured by using transmissionelectron microscope H-9000 manufactured by Hitachi, Ltd. as thetransmission electron microscope, and image analysis software KS-400manufactured by Carl Zeiss as the image analysis software, unlessotherwise noted.

As a method of collecting a sample powder from the magnetic recordingmedium in order to measure the particle size, a method disclosed in aparagraph of 0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter. The average plate ratio refers to an arithmeticaverage of (maximum long diameter/thickness or height). In a case of thedefinition (3), the average particle size is an average diameter (alsoreferred to as an average particle diameter).

In one aspect, the shape of the ferromagnetic particles configuring theferromagnetic powder contained in the magnetic layer can be a planarshape. Hereinafter, the ferromagnetic powder including the plate-shapedferromagnetic particles is referred to as a plate-shaped ferromagneticpowder. An average plate ratio of the plate-shaped ferromagnetic powdercan be preferably 2.5 to 5.0. As the average plate ratio increases,uniformity of the alignment state of the ferromagnetic particlesconfiguring the plate-shaped ferromagnetic powder tends to easilyincrease by the alignment process, and the value of ΔN tends toincrease.

As an index for a particle size of the ferromagnetic powder, anactivation volume can also be used. The “activation volume” is a unit ofmagnetization reversal. Regarding the activation volume described in theinvention and the specification, magnetic field sweep rates of acoercivity He measurement part at time points of 3 minutes and 30minutes are measured by using a vibrating sample magnetometer in anenvironment of an atmosphere temperature of 23° C.±1° C., and theactivation volume is a value obtained from the following relationalexpression of He and an activation volume V.

Hc=2Ku/Ms{1−[(kT/KuV)ln(At/0.693)]^(1/2)}

[in the expression, Ku: anisotropy constant, Ms: saturationmagnetization, k: Boltzmann's constant, T: absolute temperature, V:activation volume, A: spin precession frequency, and t: magnetic fieldreversal time]

From a viewpoint of improving the recording density, the activationvolume of the ferromagnetic powder is preferably 2,500 nm³ or smaller,more preferably 2,300 nm³ or smaller, and even more preferably 2,000 nm³or smaller. Meanwhile, from a viewpoint of stability of magnetization,the activation volume of the ferromagnetic powder is, for example,preferably 800 nm3 or greater, more preferably 1,000 nm3 or greater, andeven more preferably 1,200 nm3 or greater.

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50% to 90% by mass and more preferably 60%to 90% by mass. Components other than the ferromagnetic powder of themagnetic layer are at least a binding agent or an oxide abrasive, mayoptionally include one or more additional additives. A high fillingpercentage of the ferromagnetic powder in the magnetic layer ispreferable from a viewpoint of improvement of recording density.

(Binding Agent and Curing Agent)

The magnetic tape is a coating type magnetic tape and contains a bindingagent in the magnetic layer. The binding agent is one or more kinds ofresin. The resin may be a homopolymer or a copolymer. As the bindingagent contained in the magnetic layer, a resin selected from apolyurethane resin, a polyester resin, a polyamide resin, a vinylchloride resin, an acrylic resin obtained by copolymerizing styrene,acrylonitrile, or methyl methacrylate, a cellulose resin such asnitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylalresin such as polyvinyl acetal or polyvinyl butyral can be used alone ora plurality of resins can be mixed with each other to be used. Amongthese, a polyurethane resin, an acrylic resin, a cellulose resin, and avinyl chloride resin are preferable. These resins can be used as thebinding agent even in the non-magnetic layer and/or a back coating layerwhich will be described later. For the binding agent described above,description disclosed in paragraphs 0029 to 0031 of JP2010-024113A canbe referred to. The binding agent may be a radiation curable resin suchas an electron beam curable resin. For details of the radiation curableresin, descriptions disclosed in paragraphs 0044 to 0045 ofJP2011-048878A can be referred to.

An average molecular weight of the resin used as the binding agent canbe, for example, 10,000 to 200,000 as a weight-average molecular weight.The weight-average molecular weight of the invention and thespecification is a value obtained by performing polystyrene conversionof a value measured by gel permeation chromatography (GPC). Asmeasurement conditions, the following conditions can be used. Theweight-average molecular weight shown in examples which will bedescribed later is a value obtained by performing polystyrene conversionof a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In one aspect, as the binding agent, a binding agent containing anacidic group can be used. The acidic group of the invention and thespecification is used as a meaning including a state of a group capableof emitting H⁺ in water or a solvent including water (aqueous solvent)to dissociate anions and a salt thereof. Specific examples of the acidicgroup include a sulfonic acid group, a sulfuric acid group, a carboxygroup, a phosphoric acid group, and salts thereof. For example, a saltof a sulfonic acid group (—SO₃H) is represented by —SO₃M, and Mrepresents a group representing an atom (for example, alkali metal atomor the like) which may be cations in water or in an aqueous solvent. Thesame applies to aspects of salts of various groups described above. Asan example of the binding agent containing the acidic group, a resinincluding at least one kind of acidic group selected from the groupconsisting of a sulfonic acid group and a salt thereof (for example, apolyurethane resin or a vinyl chloride resin) can be used. However, theresin contained in the magnetic layer is not limited to these resins. Inaddition, in the binding agent containing the acidic group, a content ofthe acidic group can be, for example, 20 to 500 eq/ton. eq indicatesequivalent and SI unit is a unit not convertible. The content of variousfunctional groups such as the acidic group contained in the resin can beobtained by a well-known method in accordance with the kind of thefunctional group. As the binding agent having a great content of theacidic group is used, the value of ΔN tends to increase. The amount ofthe binding agent used in a magnetic layer forming composition can be,for example, 1.0 to 30.0 parts by mass, and preferably 1.0 to 20.0 partsby mass with respect to 100.0 parts by mass of the ferromagnetic powder.As the amount of the binding agent used with respect to theferromagnetic powder increases, the value of ΔN tends to increase.

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent. As the curing agent, in oneaspect, a thermosetting compound which is a compound in which a curingreaction (crosslinking reaction) proceeds due to heating can be used,and in another aspect, a photocurable compound in which a curingreaction (crosslinking reaction) proceeds due to light irradiation canbe used. At least a part of the curing agent may be contained in themagnetic layer in a state of being reacted (crosslinked) with othercomponents such as the binding agent, by proceeding the curing reactionin the magnetic layer forming step. This point is the same as regardinga layer formed by using a composition, in a case where the compositionused for forming the other layer contains the curing agent. Thepreferred curing agent is a thermosetting compound, polyisocyanate issuitable. For details of the polyisocyanate, descriptions disclosed inparagraphs 0124 and 0125 of JP2011-216149A can be referred to, forexample. The amount of the curing agent can be, for example, 0 to 80.0parts by mass with respect to 100.0 parts by mass of the binding agentin the magnetic layer forming composition, and is preferably 50.0 to80.0 parts by mass, from a viewpoint of improvement of strength of themagnetic layer.

(Oxide Abrasive)

The magnetic tape contains the oxide abrasive in the magnetic layer. Theoxide abrasive is a non-magnetic oxide material powder having a Mohshardness of greater than 8 and is preferably a non-magnetic oxidematerial powder having a Mohs hardness of 9 or greater. The maximum Mohshardness is 10. The oxide abrasive may be an inorganic oxide powder oran organic oxide powder, and is preferably an inorganic oxide powder.Specifically, examples of abrasives include powders of alumina (Al₂O₃),titanium oxide (TiO₂), cerium oxide (CeO₂), and zirconium oxide (ZrO₂),and among these, an alumina powder is preferable. The Mohs hardness ofalumina is about 9. For details of the alumina powder, descriptionsdisclosed in paragraph 0021 of JP2013-229090A can be referred to. Thespecific surface area can be used as an index of the particle size ofthe oxide abrasive. It is considered that the larger the specificsurface area, the smaller the particle size of the primary particleconstituting the oxide abrasive. As the oxide abrasive, it is preferableto use an oxide abrasive having a specific surface area (hereinafter,also referred to as a “BET specific surface area”) of 14 m²/g or greatermeasured by a brunauer-emmett-teller (BET) method. From the viewpoint ofdispersibility, it is preferable to use an oxide abrasive having a BETspecific surface area of 40 m²/g or smaller. The content of the oxideabrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass,and more preferably 1.0 to 10.0 parts by mass with respect to 100.0parts by mass of the ferromagnetic hexagonal powder.

(Additives)

The magnetic layer may contain the ferromagnetic powder, the bindingagent, and the oxide abrasive, and may further contain one or moreadditives as necessary. As the additives, the curing agent describedabove is used as an example. In addition, examples of the additive thatmay be contained in the magnetic layer include non-magnetic powder otherthan the oxide abrasive, a lubricant, a dispersing agent, a dispersingassistant, an antibacterial agent, an antistatic agent, and anantioxidant. As the additives, a commercially available product can besuitably selected according to the desired properties or manufactured bya well-known method, and can be used with any amount. For example, forthe lubricant, a description disclosed in paragraphs 0030 to 0033, 0035,and 0036 of JP2016-126817A can be referred to. The non-magnetic layermay contain the lubricant. For the lubricant which may be contained inthe non-magnetic layer, a description disclosed in paragraphs 0030,0031, 0034, 0035, and 0036 of JP2016-126817A can be referred to. For thedispersing agent, a description disclosed in paragraphs 0061 and 0071 ofJP2012-133837A can be referred to. The dispersing agent may be includedin the non-magnetic layer. For the dispersing agent which may becontained in the non-magnetic layer, a description disclosed in aparagraph 0061 of JP2012-133837A can be referred to.

Moreover, as the dispersing agent, the dispersing agent for improvingthe dispersibility of the oxide abrasive can be exemplified. Examples ofthe compound capable of functioning as such a dispersing agent includearomatic hydrocarbon compounds having a phenolic hydroxy group.“Phenolic hydroxy group” refers to a hydroxy group directly bonded to anaromatic ring. The aromatic ring contained in the aromatic hydrocarboncompound may be a single ring, a polycyclic structure, or a fused ring.From the viewpoint of improving the dispersibility of the abrasive, anaromatic hydrocarbon compound containing a benzene ring or a naphthalenering is preferable. The aromatic hydrocarbon compound may have asubstituent other than the phenolic hydroxy group. Examples of thesubstituent other than the phenolic hydroxy group include a halogenatom, an alkyl group, an alkoxy group, an amino group, an acyl group, anitro group, a nitroso group, and a hydroxyalkyl group, and a halogenatom, an alkyl group, an alkoxy group, an amino group, and ahydroxyalkyl group are preferable. The number of phenolic hydroxy groupscontained in one molecule of the aromatic hydrocarbon compound may beone, two, three, or more.

As the preferable aspect of the aromatic hydrocarbon compound having aphenolic hydroxy group, a compound represented by General Formula 100below can be exemplified.

[In General Formula 100, two of X¹⁰¹ to X¹⁰⁸ are a hydroxy group, andthe other six each independently represent a hydrogen atom or asubstituent.]

In the compound represented by General Formula 100, the substitutionpositions of two hydroxy groups (phenolic hydroxy groups) are notparticularly limited.

In the compound represented by General Formula 100, two of X¹⁰¹ to X¹⁰⁸are hydroxy groups (phenolic hydroxy groups), and the other six eachindependently represent a hydrogen atom or a substituent. Further, inX¹⁰¹ to X¹⁰⁸, all of the portions other than the two hydroxy groups maybe hydrogen atoms, or a part or all of them may be a substituent.Examples of the substituent include the substituents described above. Asa substituent other than the two hydroxy groups, one or more phenolichydroxy groups may be contained. From the viewpoint of improving thedispersibility of the abrasive, it is preferable that other than the twohydroxy groups of X¹⁰¹ to X¹⁰⁸ are not phenolic hydroxy groups. Statedanother way, the compound represented by General Formula 100 ispreferably dihydroxynaphthalene or a derivative thereof, and morepreferably 2,3-dihydroxynaphthalene or a derivative thereof. Preferableexamples of the substituent represented by X¹⁰¹ to X¹⁰⁸ include ahalogen atom (for example, a chlorine atom and a bromine atom), an aminogroup, an alkyl group having 1 to 6 carbon atoms (preferably 1 to 4), amethoxy group, and an ethoxy group, an acyl group, a nitro group, and anitroso group, and a —CH₂OH group.

For the dispersing agent for enhancing the dispersibility of the oxideabrasive, paragraphs 0024 to 0028 of JP2014-179149A can be referred to.

The dispersing agent for enhancing the dispersibility of the oxideabrasive can be used in a proportion of 0.5 to 20.0 parts by mass forexample, and is preferably used in a proportion of 1.0 to 10.0 parts bymass with respect to 100.0 parts by mass of the abrasive at the time ofpreparing the magnetic layer forming composition (preferably at the timeof preparing the abrasive solution as described below).

As the non-magnetic powder other than the oxide abrasive that can becontained in the magnetic layer, a non-magnetic powder that cancontribute to friction characteristic control by forming projections onthe surface of the magnetic layer (hereinafter also referred to as a“projection forming agent”) can be exemplified. As the projectionforming agent, various non-magnetic powders generally used as aprojection forming agent in the magnetic layer can be used. These may bean inorganic substance powder (an inorganic powder) or an organicsubstance powder (an organic powder). In one aspect, from the viewpointof uniform frictional characteristics, the particle diameterdistribution of the projection forming agent is preferably monodisperseshowing a single peak, not monodisperse having a plurality of peaks inthe distribution. From the viewpoint of easy availability of themonodisperse particles, the projection forming agent is preferably aninorganic powder. Examples of the inorganic powder can include powder ofmetal oxide, metal carbonate, metal sulfate, metal nitride, metalcarbide, and metal sulfide. The particles constituting the projectionforming agent (a non-magnetic powder other than the oxide abrasive) arepreferably colloidal particles, and more preferably inorganic oxidecolloidal particles. Further, from the viewpoint of availability ofmonodisperse particles, the inorganic oxide constituting the inorganicoxide colloidal particles is preferably silicon dioxide (silica). Theinorganic oxide colloidal particles are more preferably colloidal silica(silica colloidal particles). In the invention and the specification,“colloid particles” refers to particles that can be dispersed withoutsettling to yield a colloidal dispersion in a case of being added 1 gper 100 mL of at least one organic solvent of methyl ethyl ketone,cyclohexanone, toluene, ethyl acetate, or a mixed solvent containing twoor more of the above solvents in an any mixing ratio. In another aspect,the projection forming agent is preferably carbon black. The averageparticle size of the projection forming agent can be, for example, 30 to300 nm, and preferably 40 to 200 nm. In addition, from the viewpoint ofexhibiting better function of the projection forming agent, the contentof the projection forming agent in the magnetic layer is preferably 1.0to 4.0 parts by mass, and more preferably 1.5 to 3.5 parts by mass withrespect to 100.0 parts by mass of the ferromagnetic powder.

The magnetic layer described above can be provided on the surface of thenon-magnetic support directly or indirectly through the non-magneticlayer.

<Non-Magnetic Layer>

Next, the non-magnetic layer will be described. The magnetic tape maycontain a magnetic layer directly on the surface of the non-magneticsupport or may include a non-magnetic layer including the non-magneticpowder and the binding agent between the non-magnetic support and themagnetic layer. The non-magnetic powder contained in the non-magneticlayer may be an inorganic powder or an organic powder. In addition,carbon black and the like can be used. Examples of the inorganic powderinclude powder of metal, metal oxide, metal carbonate, metal sulfate,metal nitride, metal carbide, and metal sulfide. The non-magnetic powdercan be purchased as a commercially available product or can bemanufactured by a well-known method. For details thereof, descriptionsdisclosed in paragraphs 0036 to 0039 of JP2010-024113A can be referredto. The content (filling percentage) of the non-magnetic powder of thenon-magnetic layer is preferably 50% to 90% by mass and more preferably60% to 90% by mass.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

The non-magnetic layer of the invention and the specification alsoincludes a substantially non-magnetic layer containing a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density of 10 mT or smaller, alayer having coercivity of 7.96 kA/m (100 Oe) or smaller, or a layerhaving a residual magnetic flux density of 10 mT or smaller andcoercivity of 7.96 kA/m (100 Oe) or smaller. It is preferable that thenon-magnetic layer does not have a residual magnetic flux density andcoercivity.

<Non-Magnetic Support>

Next, the non-magnetic support (hereinafter, also simply referred to asa “support”) will be described.

As the non-magnetic support, well-known components such as polyethyleneterephthalate, polyethylene naphthalate, polyamide, polyamide imide,aromatic polyamide subjected to biaxial stretching are used. Amongthese, polyethylene terephthalate, polyethylene naphthalate, andpolyamide are preferable. Corona discharge, plasma treatment,easy-bonding treatment, or heat treatment may be performed with respectto these supports in advance.

<Back Coating Layer>

The magnetic tape can also include a back coating layer containing anon-magnetic powder and a binding agent on a surface of the non-magneticsupport opposite to the surface provided with the magnetic layer. Theback coating layer preferably contains any one or both of carbon blackand inorganic powder. In regards to the binding agent contained in theback coating layer and various additives which can be randomly containedtherein, a well-known technology regarding the back coating layer can beapplied, and a well-known technology regarding the list of the magneticlayer and/or the non-magnetic layer can also be applied. For example,for the back coating layer, descriptions disclosed in paragraphs 0018 to0020 of JP2006-331625A and page 4, line 65, to page 5, line 38, of U.S.Pat. No. 7,029,774B can be referred to.

<Various Thicknesses>

The thicknesses of the non-magnetic support and each layer of themagnetic recording medium will be described below.

The thickness of the non-magnetic support is, for example, 3.0 to 80.0μm, preferably 3.0 to 50.0 μm, and more preferably 3.0 to 10.0 μm.

A thickness of the magnetic layer can be optimized according to asaturation magnetization of a magnetic head used, a head gap length, arecording signal band, and the like. The thickness of the magnetic layeris normally 10 nm to 100 nm, and is preferably 20 to 90 nm and morepreferably 30 to 70 nm, from a viewpoint of realization of high-densityrecording. The magnetic layer may be at least one layer, or the magneticlayer can be separated to two or more layers having magnetic properties,and a configuration regarding a well-known multilayered magnetic layercan be applied. A thickness of the magnetic layer which is separatedinto two or more layers is a total thickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.1 to 1.5 μm andpreferably 0.1 to 1.0 μm.

A thickness of the back coating layer is preferably 0.9 μm or smallerand even more preferably 0.1 to 0.7 μm.

The thicknesses of various layers and the non-magnetic support areobtained by exposing a cross section of the magnetic tape in a thicknessdirection by a well-known method of ion beams or microtome, andobserving the exposed cross section with a scanning transmissionelectron microscope (STEM). For the specific examples of the measurementmethod of the thickness, a description disclosed regarding themeasurement method of the thickness in examples which will be describedlater can be referred to.

<Manufacturing Step>

(Preparation of Each Layer Forming Composition)

Steps of preparing the composition for forming the magnetic layer, thenon-magnetic layer, or the back coating layer generally include at leasta kneading step, a dispersing step, or a mixing step provided before andafter these steps, if necessary. Each step may be divided into two ormore stages. The components used in the preparation of each layerforming composition may be added at an initial stage or in a middlestage of each step. As the solvent, one kind or two or more kinds ofvarious solvents generally used for manufacturing a coating typemagnetic recording medium can be used. For the solvent, a descriptiondisclosed in a paragraph 0153 of JP2011-216149A can be referred to, forexample. In addition, each component may be separately added in two ormore steps. For example, the binding agent may be separately added inthe kneading step, the dispersing step, and a mixing step for adjustinga viscosity after the dispersion. In order to manufacture the magnetictape, a well-known manufacturing technology of the related art can beused in various steps. In the kneading step, an open kneader, acontinuous kneader, a pressure kneader, or a kneader having a strongkneading force such as an extruder is preferably used. For details ofthe kneading processes, descriptions disclosed in JP1989-106338A(JP-H01-106338A) and JP1989-079274A (JP-H01-079274A) can be referred to.As a disperser, a well-known disperser can be used. In any stage ofpreparing each layer forming composition, the filtration may beperformed by a well-known method. The filtration can be performed byusing a filter, for example. As the filter used in the filtration, afilter having a hole diameter of 0.01 to 3 μm (for example, filter madeof glass fiber or filter made of polypropylene) can be used, forexample.

The FIB abrasive diameter tends to decrease as the oxide abrasive ispresent in a finer state in the magnetic layer. One means for allowingthe oxide abrasive to be present in a finer state in the magnetic layeris to use a dispersing agent capable of enhancing the dispersibility ofthe oxide abrasive as described above. In addition, in order to make theoxide abrasive in a finer state in the magnetic layer, it is preferablethat an abrasive having a small particle size is used, and the abrasiveis suppressed from being aggregated, suppressed from being unevenlydistributed, and made to be uniformly dispersed in the magnetic layer.One means for this is to strengthen the dispersion conditions of theoxide abrasive during preparation of the magnetic layer formingcomposition. For example, separately dispersing the oxide abrasive fromthe ferromagnetic powder is one aspect of strengthening the dispersioncondition. The separate dispersion is specifically a method of preparinga magnetic layer forming composition through a step of mixing anabrasive solution containing an oxide abrasive and a solvent (here,ferromagnetic powder is not substantially included) with a magneticliquid contained the ferromagnetic powder, a solvent, and a bindingagent. Thus, dispersibility of the oxide abrasive in the magnetic layerforming composition can be improved by separately dispersing and mixingthe oxide abrasive and the ferromagnetic powder. The expression“ferromagnetic powder is not substantially contained” means that theferromagnetic powder is not added as a constituent component of theabrasive solution, and a small amount of the ferromagnetic powder mixedas impurities without any intention is allowed. In addition to or withother separate dispersion, means such as long-time dispersionprocessing, by arbitrarily combining use of small-sized dispersion media(for example, reducing the diameter of dispersed beads in beaddispersion), or increasing the density of dispersion media in adisperser, the dispersion condition can be strengthened. Commerciallyavailable dispersers and dispersion media can be used. Further, thecentrifugal treatment of the abrasive solution contributes to causingthe oxide abrasive to be present in the magnetic layer in a finer stateby removing oxide particles larger than the average particle size and/oraggregated particles among the particles constituting the oxideabrasive. Centrifugal treatment can be performed using a commerciallyavailable centrifuge. Further, it is preferable to filter the abrasivesolution by filter filtration or the like in order to remove coarseaggregates in which particles constituting the oxide abrasive areaggregated. Removing such coarse aggregates can also contribute to thepresence of the oxide abrasive in a finer state in the magnetic layer.For example, filter filtration using a filter having a smaller holediameter can contribute to the presence of the oxide abrasive in a finerstate in the magnetic layer. Also, by adjusting the various processingconditions (for example, stirring conditions, dispersion processingconditions, or filtration conditions) after mixing the abrasive solutionwith the components for preparing the magnetic layer forming compositionsuch as the ferromagnetic powder, the dispersibility of the oxideabrasive in the magnetic layer forming composition can be improved. Thiscan also contribute to the presence of the oxide abrasive in a finerstate in the magnetic layer. However, in a case where the oxide abrasiveis present in a very fine state in the magnetic layer, the FIB abrasivediameter becomes 0.08 μm or smaller, and thus it is preferable to adjustthe various conditions for preparing the abrasive solution so that a FIBabrasive diameter of 0.14 μm or smaller can be realized.

Regarding ΔN, as a period of the dispersion time of the magnetic liquidincreases, the value of ΔN tends to increase. This is thought that, as aperiod of the dispersion time of the magnetic liquid increases, thedispersibility of the ferromagnetic powder in the coating layer of themagnetic layer forming composition increases, and the uniformity of thealignment state of the ferromagnetic particles configuring theferromagnetic powder by the alignment process tends to easily increase.

In addition, as the period of the dispersion time in a case of mixingand dispersing various components of the non-magnetic layer formingcomposition increases, the value of ΔN tends to increase.

The dispersion time of the magnetic liquid and the dispersion time ofthe non-magnetic layer forming composition described above need only beset so that ΔN of 0.25 to 0.40 can be realized.

(Coating Step)

The non-magnetic layer and the magnetic layer can be formed byperforming multilayer coating with the non-magnetic layer formingcomposition and the magnetic layer forming composition in order or atthe same time. The back coating layer can be formed by applying the backcoating layer forming composition onto the surface of the non-magneticsupport opposite to the surface provided with the non-magnetic layer andthe magnetic layer (or non-magnetic layer and/or the magnetic layer isto be provided). In addition, the coating step for forming each layercan be also performed by being divided into two or more stages. Forexample, in one aspect, the magnetic layer forming composition can beapplied in two or more stages. In this case, a drying process may beperformed or may not be performed during the coating steps of twostages. In addition, the alignment process may be performed or may notbe performed during the coating steps of two stages. For details of thecoating for forming each layer, a description disclosed in a paragraph0066 of JP2010-231843A can be referred to. In addition, for the dryingstep after applying each layer forming composition, a well-knowntechnology can be applied. Regarding the magnetic layer formingcomposition, as a drying temperature of a coating layer which is formedby applying the magnetic layer forming composition (hereinafter, alsoreferred to as a “coating layer of the magnetic layer formingcomposition” or simply a “coating layer”) decreases, the value of ΔNtends to increase. The drying temperature can be an atmospheretemperature for performing the drying step, for example, and need onlybe set so that ΔN of 0.25 to 0.40 can be realized.

(Other Steps)

For various other steps for manufacturing the magnetic tape, awell-known technology can be applied. For details of the various steps,descriptions disclosed in paragraphs 0067 to 0070 of JP2010-231843A canbe referred to, for example.

For example, it is preferable to perform the alignment process withrespect to the coating layer of the magnetic layer forming compositionwhile the coating layer is wet. From a viewpoint of ease of exhibitingof ΔN of 0.25 to 0.40, the alignment process is preferably performed bydisposing a magnet so that a magnetic field is vertically applied to thesurface of the coating layer of the magnetic layer forming composition(that is, homeotropic alignment process). The strength of the magneticfield during the alignment process need only be set so that ΔN of 0.25to 0.40 can be realized. In addition, in a case of performing thecoating step of the magnetic layer forming composition by the coatingsteps of two or more stages, it is preferable to perform the alignmentprocess at least after the final coating step, and it is more preferableto perform the homeotropic alignment process. For example, in a case offorming the magnetic layer by the coating steps of two stages, thedrying step is performed without performing the alignment process afterthe coating step in the first stage, and then, the alignment process canbe performed with respect to the formed coating layer in the coatingstep in the second stage.

In addition, it is preferable to perform the calender process in anystage after drying the coating layer of the magnetic layer formingcomposition. For the conditions of the calender process, a descriptiondisclosed in a paragraph 0026 of JP2010-231843A can be referred to. Asthe calender temperature (surface temperature of the calender roll)increases, the value of ΔN tends to increase. The calender temperatureneed only be set so that ΔN of 0.25 to 0.40 can be realized.

As described above, it is possible to obtain the magnetic tape accordingto one aspect of the invention. The magnetic tape is normallyaccommodated in a magnetic tape cartridge and the magnetic tapecartridge is mounted on a magnetic recording and reproducing device. Aservo pattern can also be formed in the magnetic tape by a well-knownmethod, in order to allow head tracking servo to be performed in themagnetic recording and reproducing device. In a case of reproducinginformation recorded on the magnetic tape in the magnetic recording andreproducing device in the low temperature and low humidity environment,it is possible to decrease the generation frequency of the missingpulse, in a case of using the magnetic tape according to one aspect ofthe invention.

[Magnetic Recording and Reproducing Device]

Another aspect of the invention relates to a magnetic recording andreproducing device including the magnetic tape and a magnetic head.

In the invention and the specification, the “magnetic recording andreproducing device” means a device capable of performing at least one ofthe recording of information on the magnetic tape or the reproducing ofinformation recorded on the magnetic tape. Such a device is generallycalled a drive. The magnetic head included in the magnetic recording andreproducing device can be a recording head capable of performing therecording of information on the magnetic tape, and can also be areproducing head capable of performing the reproducing of informationrecorded on the magnetic tape. In addition, in one aspect, the magneticrecording and reproducing device can include both of a recording headand a reproducing head as separate magnetic heads. In another aspect,the magnetic head included in the magnetic recording and reproducingdevice can also have a configuration of comprising both of a recordingelement and a reproducing element in one magnetic head. As thereproducing head, a magnetic head (MR head) including a magnetoresistive(MR) element capable of reading information recorded on the magnetictape with excellent sensitivity as the reproducing element ispreferable. As the MR head, various well-known MR heads can be used. Inaddition, the magnetic head which performs the recording of informationand/or the reproducing of information may include a servo patternreading element. Alternatively, as a head other than the magnetic headwhich performs the recording of information and/or the reproducing ofinformation, a magnetic head (servo head) comprising a servo patternreading element may be included in the magnetic recording andreproducing device.

In the magnetic recording and reproducing device, the recording ofinformation on the magnetic tape and the reproducing of informationrecorded on the magnetic tape can be performed by bringing the surfaceof the magnetic layer of the magnetic tape into contact with themagnetic head and sliding. The magnetic recording and reproducing devicemay include the magnetic tape according to one aspect of the invention,and well-known technologies can be applied for the other configurations.

The magnetic recording and reproducing device includes the magnetic tapeaccording to one aspect of the invention. Therefore, it is possible todecrease the generation frequency of the missing pulse in a case ofreproducing information recorded on the magnetic tape in the lowtemperature and low humidity environment. In addition, even in a casewhere the surface of the magnetic layer and the head slide on each otherfor recording information on the magnetic tape in the low temperatureand low humidity environment, it is possible to suppress the unstablecontact state of the surface of the magnetic layer and the head due tothe head attached material caused by the chipping of the surface of themagnetic layer and/or the head chipping.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description are based on mass.

Example 1

<Preparation of Abrasive Solution>

With respect to 100.0 part of the oxide abrasive (an alumina powder)shown in Table 1, 2,3-dihydroxynaphthalene in the amount shown in Table1 (manufactured by Tokyo Chemical Industry Co., Ltd.), 31.3 parts of a32% solution of a polyester polyurethane resin having a SO₃Na group as apolar group (UR-4800 manufactured by Toyobo Co., Ltd. (polar groupamount: 80 meq/kg)) (solvent is a mixed solvent of methyl ethyl ketoneand toluene), and 570.0 parts of a mixed solution of methyl ethyl ketoneand cyclohexanone 1:1 (the mass ratio) as a solvent were mixed anddispersed by a paint shaker for the time shown in Table 1 (beaddispersion time) in the presence of zirconia beads (the bead diameter:0.1 mm). After the dispersion, the centrifugal treatment was performedwith respect to the obtained dispersion liquid by separating thedispersion liquid and the beads by the mesh. The centrifugal treatmentwas performed with CS150GXL manufactured by Hitachi Koki Co., Ltd. (theused rotor was S100AT6 manufactured by the same company) as a centrifugefor the times shown in Table 1 (centrifugation time) at the rotationspeed (rpm; rotation per minute) shown in Table 1. Then, filtration wasperformed with the filter having the hole diameter shown in Table 1, andthe alumina dispersion (the abrasive solution) was obtained.

<Preparation of Magnetic Layer Forming Composition>

(Magnetic Liquid)

Plate-shaped ferromagnetic hexagonal barium ferrite powder: 100.0 parts

(Activation volume and average plate ratio: see Table 1)

SO₃Na group-containing polyurethane resin: see Table 1

(Weight-average molecular weight: 70,000, SO₃Na group: see Table 1)

Cyclohexanone: 150.0 parts

Methyl ethyl ketone: 150.0 parts

(Abrasive Solution)

Alumina dispersion prepared as described above: 6.0 parts

(Silica Sol (projection forming agent liquid))

Colloidal silica (Average particle size: 100 nm): 2.0 parts

Methyl ethyl ketone: 1.4 parts

(Other Components)

Stearic acid: 2.0 parts

Butyl stearate: 2.0 parts

Polyisocyanate (CORONATE (registered trademark) manufactured by TosohCorporation): 2.5 parts

(Finishing Additive Solvent)

Cyclohexanone: 200.0 parts

Methyl ethyl ketone: 200.0 parts

(Preparation Method)

The magnetic liquid was prepared by beads-dispersing of variouscomponents of the magnetic liquid described above by using beads as thedispersion medium in a batch type vertical sand mill. The beaddispersion was performed using zirconia beads (the bead diameter: seeTable 1) as the beads for the time shown in Table 1 (the magnetic liquidbead dispersion time).

The obtained magnetic liquid, the abrasive solution, silica sol, othercomponents and finishing additive solvent were introduced into adissolver stirrer and stirred at a circumferential speed of 10 m/sec forthe time (the stirring time) shown in Table 1. Then, after performingultrasonic dispersion processing for a time shown in Table 1 (theultrasonic dispersion processing time) at a flow rate of 7.5 kg/minusing a flow type ultrasonic disperser, the filtration was performed forthe number of times shown in Table 1 with a filter having a holediameter shown in Table 1 to prepare the magnetic layer formingcomposition.

<Preparation of Non-Magnetic Layer Forming Composition>

Each component among various components of the non-magnetic layerforming composition shown below excluding stearic acid, butyl stearate,cyclohexanone, and methyl ethyl ketone was beads-dispersed (dispersionmedium: zirconia beads (the bead diameter: 0.1 mm), dispersion time: seeTable 1) by using a batch type vertical sand mill to obtain a dispersionliquid. After that, the remaining components were added into theobtained dispersion liquid and stirred with a dissolver stirrer. Then,the obtained dispersion liquid was filtered with a filter (holediameter: 0.5 μm) and a non-magnetic layer forming composition wasprepared.

Non-magnetic inorganic powder: α-iron oxide: 100.0 parts

Average particle size (average long axis length): 0.15 μm

Average acicular ratio: 7

BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

Average particle size: 20 nm

Electron beam curable vinyl chloride copolymer: 13.0 parts

Electron beam curable polyurethane resin: 6.0 parts

Stearic acid: 1.0 parts

Butyl stearate: 1.0 parts

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

<Preparation of Back Coating Layer Forming Composition>

Each component among various components of the back coating layerforming composition shown below excluding stearic acid, butyl stearate,polyisocyanate, and cyclohexanone was kneaded and diluted by an openkneader, and a mixed solution was obtained. After that, the obtainedmixed solution was subjected to a dispersion processing of 12 passes,with a transverse beads mill and zirconia beads having a bead diameterof 1.0 mm, by setting a bead filling percentage as 80 volume %, acircumferential speed of rotor distal end as 10 msec, and a retentiontime for 1 pass as 2 minutes. After that, the remaining components wereadded into the obtained dispersion liquid and stirred with a dissolverstirrer. Then, the obtained dispersion liquid was filtered with a filter(hole diameter: 1.0 μm) and a back coating layer forming composition wasprepared.

Non-magnetic inorganic powder: α-iron oxide: 80.0 parts

Average particle size (average long axis length): 0.15 μm

Average acicular ratio: 7

BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

Average particle size: 20 nm

A vinyl chloride copolymer: 13.0 parts

A sulfonic acid salt group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Methyl ethyl ketone: 155.0 parts

Stearic acid: 3.0 parts

Butyl stearate: 3.0 parts

Polyisocyanate: 5.0 parts

Cyclohexanone: 355.0 parts

<Manufacturing of Magnetic Tape>

The non-magnetic layer forming composition was applied onto apolyethylene naphthalate support and dried, and then the non-magneticlayer was formed by irradiating an electron beam so that the energy was40 kGy at an acceleration voltage of 125 kV.

The formed magnetic layer forming composition was applied on the surfaceof the non-magnetic layer so that the thickness after drying becomes 50nm to form the coating layer.

The homeotropic alignment process and the drying process were performedby applying a magnetic field having a strength shown in Table 1 in thevertical direction with respect to the surface of the coating layerusing opposing magnet in the atmosphere at the atmosphere temperatureshown in Table 1 (the drying temperature) while the coating layer waswet, and the magnetic layer was formed.

Thereafter, a back coating layer forming composition was applied on thesurface of the support opposite to the surface on which the non-magneticlayer and the magnetic layer were formed, and dried.

After that, a surface smoothing treatment (a calender process) wasperformed with a calender roll configured of only a metal roll, at acalendering speed of 80 m/min, linear pressure of 300 kg/cm (294 kN/m),and a calender temperature (a surface temperature of a calender roll)shown in Table 1.

Then, a heat treatment was performed in the environment of theatmosphere temperature of 70° C. for 36 hours. After the heat treatment,slits were made ½ inch (0.0127 meters) side, and the surface of themagnetic layer was cleaned by the tape cleaning device attached to adevice having a device for sending and winding slit products so that thenon-woven fabric and razor blade were pressed against the magnetic layersurface. Thereafter, a servo pattern was formed on the magnetic layer bya commercially available servo writer.

By doing so, a magnetic tape of Example 1 was manufactured.

Examples 2, 3, 5 and Comparative Examples 1 to 8

A magnetic tape was manufactured in the same manner as in Example 1,except that various conditions shown in Table 1 were changed as shown inTable 1. All of the oxide abrasives listed in Table 1 are aluminapowders.

In Table 1, in the comparative examples in which “no alignment process”is shown in the column of “formation and alignment of magnetic layer”,the magnetic tape was manufactured without performing the alignmentprocess regarding the coating layer of the magnetic layer formingcomposition.

Example 4

After forming the non-magnetic layer, the magnetic layer formingcomposition was applied on the surface of the non-magnetic layer so thatthe thickness after drying becomes 25 nm to form a first coating layer.The first coating layer was passed through the atmosphere at theatmosphere temperature shown in Table 1 (the drying temperature) withoutapplication of a magnetic field to form a first magnetic layer (noalignment process).

After that, the magnetic layer forming composition was applied on thesurface of the first magnetic layer so that the thickness after dryingbecomes 25 nm to form a second coating layer. The homeotropic alignmentprocess and the drying process were performed by applying a magneticfield having a strength shown in Table 1 in the vertical direction withrespect to the surface of the second coating layer using opposing magnetin the atmosphere at the atmosphere temperature shown in Table 1 (thedrying temperature) while the second coating layer was wet, and a secondmagnetic layer was formed.

A magnetic tape was manufactured in the same manner as in Example 1,except that the multilayered magnetic layer was formed as describedabove.

Comparative Example 9

After forming the non-magnetic layer, the magnetic layer formingcomposition was applied on the surface of the non-magnetic layer so thatthe thickness after drying becomes 25 nm to form a first coating layer.The homeotropic alignment process and the drying process were performedby applying a magnetic field having a strength shown in Table 1 in thevertical direction with respect to the surface of the first coatinglayer using opposing magnet in the atmosphere at the atmospheretemperature shown in Table 1 (the drying temperature) while the firstcoating layer was wet, and a first magnetic layer was formed.

After that, the magnetic layer forming composition was applied on thesurface of the first magnetic layer so that the thickness after dryingbecomes 25 mu to form a second coating layer. The second coating layerwas passed through the atmosphere at the atmosphere temperature shown inTable 1 (the drying temperature) without application of a magnetic fieldto form a second magnetic layer (no alignment process).

A magnetic tape was manufactured in the same manner as in Example 1,except that the multilayered magnetic layer was formed as describedabove.

Comparative Example 10

After forming the non-magnetic layer, the magnetic layer formingcomposition was applied on the surface of the non-magnetic layer so thatthe thickness after drying becomes 25 nm to form a first coating layer.The homeotropic alignment process and the drying process were performedby applying a magnetic field having a strength shown in Table 1 in thevertical direction with respect to the surface of the first coatinglayer using opposing magnet in the atmosphere at the atmospheretemperature shown in Table 1 (the drying temperature) while the firstcoating layer was wet, and a first magnetic layer was formed.

After that, the magnetic layer forming composition was applied on thesurface of the first magnetic layer so that the thickness after dryingbecomes 25 nm to form a second coating layer. The second coating layerwas passed through the atmosphere at the atmosphere temperature shown inTable 1 (the drying temperature) without application of a magnetic fieldto form a second magnetic layer (no alignment process).

A magnetic tape was manufactured in the same manner as in ComparativeExample 8, except that the multilayered magnetic layer was formed asdescribed above.

Comparative Example 11

After forming the non-magnetic layer, the magnetic layer formingcomposition was applied on the surface of the non-magnetic layer so thatthe thickness after drying becomes 25 nm to form a first coating layer.The first coating layer was passed through the atmosphere at theatmosphere temperature shown in Table 1 (the drying temperature) withoutapplication of a magnetic field to form a first magnetic layer (noalignment process).

After that, the magnetic layer forming composition was applied on thesurface of the first magnetic layer so that the thickness after dryingbecomes 25 nm to form a second coating layer. The homeotropic alignmentprocess and the drying process were performed by applying a magneticfield having a strength shown in Table 1 in the vertical direction withrespect to the surface of the second coating layer using opposing magnetin the atmosphere at the atmosphere temperature shown in Table 1 (thedrying temperature) while the second coating layer was wet, and a secondmagnetic layer was formed.

A magnetic tape was manufactured in the same manner as in ComparativeExample 6, except that the multilayered magnetic layer was formed asdescribed above.

[Evaluation of Physical Properties of Magnetic Tape]

(1) FIB Abrasive Diameter

The FIB abrasive diameter of each magnetic tape manufactured wasobtained by the following method. MI4050 manufactured by HitachiHigh-Technologies Corporation was used as a focused ion beam device, andfree software ImageJ was used as image analysis software.

(i) Acquisition of Secondary Ion Image

The surface of the back coating layer of the measurement sample cut outfrom each of the manufactured magnetic tapes was attached to theadhesive layer of a commercially available carbon double-sided tape forSEM measurement (double-sided tape with a carbon film formed on analuminum substrate). The adhesive layer on the surface opposite to thesurface of the double-sided tape with the back coating layer attachedthereto was attached to the sample stage of the focused ion beam device.Thus, the measurement sample was disposed on the sample stage of thefocused ion beam device in a state where the magnetic layer surfacefaces upward.

The pre-imaging coating process was not performed, the beam setting ofthe focused ion beam device was set to an acceleration voltage of 30 kV,a current value of 133 pA, a beam size of 30 nm, and brightness of 50%,and the SI signal was detected by the secondary ion detector. Byperforming the ACB at three unimaged regions on the surface of themagnetic layer, the tint of the image was stabilized, and the contrastreference value and the brightness reference value were determined. Thecontrast value obtained by lowering the contrast reference valuedetermined by the ACB by 1% and the above brightness reference valuewere set as the imaging conditions. The unimaged region on the surfaceof the magnetic layer was selected, and imaging was performed with Pixeldistance=25.0 (nm/pixel) under the imaging conditions determined above.The image capture method was PhotoScan Dot×4_Dwell Time 15 μsec (capturetime: 1 minute), and the capture size was 25 μm square. Thus, asecondary ion image of a 25 μm square region on the surface of themagnetic layer was obtained. The obtained secondary ion image was savedas a JPEG file with Export Image after right-clicking on the capturescreen after scanning. After confirming that the number of pixels of theimage was 2000 pixels×2100 pixels, the cross mark and micron bar of thecaptured image were deleted, and a 2000 pixel×2000 pixel image wasobtained.

(ii) Calculation of FIB Abrasive Diameter

The image data of the secondary ion image acquired in the section (i)was dragged and dropped onto the image analysis software ImageJ.

The color tone of the image data is changed to 8 bits using the imageanalysis software. Specifically, section “Image” on the operation menuof the image analysis software was pressed, and 8 bits in section “Type”was selected.

For binarization processing, a lower limit value of 250 gradation and anupper limit value of 255 gradation are selected, and the binarizationprocessing is executed with these two threshold values. Specifically, onthe operation menu of the image analysis software, section “Image” waspressed, “Threshold” of “Adjust” was selected, 250 as a lower limitvalue and 255 as an upper limit value were selected, and “apply” wasselected. For the obtained image, section“Process” in the operation menuof the image analysis software is pressed, “Despeckle” was selected fromsection “Noise”, Size4.0-Infinity in Analyze Particle was set, and thenoise component was removed.

For the obtained binarized image, section “Analyze Particle” wasselected from the operation menu of the image analysis software, and thenumber of portions that shine white and Area (unit: Pixel) on the imagewere obtained. The area was obtained by converting Area (unit: Pixel)into an area for each portion that shines white on the screen by imageanalysis software. Specifically, since 1 pixel corresponds to 0.0125 μmin the image obtained under the above imaging conditions, the area A[μm²] was calculated from the area A=Area pixel×0.0125{circumflex over( )}2. Using the calculated area, the equivalent circle diameter L wasobtained for each portion that shines white by the equivalent circlediameter L=(A/π){circumflex over ( )}(½)×2=L.

The above process was performed four times at different locations (25 μmsquare) on the surface of the magnetic layer of the measurement sample,and from the obtained results, the FIB abrasive diameter was calculatedby FIB abrasive diameter=Σ(Li)/Σi.

(2) Thicknesses of Non-Magnetic Support and Each Layer

The thicknesses of the magnetic layer, the non-magnetic layer, thenon-magnetic support, and the back coating layer of each manufacturedmagnetic tape were measured by the following method. As a result of themeasurement, in all of the magnetic tapes, the thickness of the magneticlayer was 50 nm, the thickness of the non-magnetic layer was 0.7 μm, thethickness of the non-magnetic support was 5.0 μm, and the thickness ofthe back coating layer was 0.5 μm.

The thicknesses of the magnetic layer, the non-magnetic layer, and thenon-magnetic support measured here were used for calculating thefollowing refractive index.

(i) Manufacturing of Cross Section Observation Sample

A cross section observation sample including all regions of the magnetictape from the magnetic layer side surface to the back coating layer sidesurface in the thickness direction was manufactured according to themethod disclosed in paragraphs 0193 and 0194 of JP2016-177851A.

(ii) Thickness Measurement

The manufactured sample was observed with the STEM and a STEM image wascaptured. This STEM image was a STEM-high-angle annular dark field(HAADF) image which is captured at an acceleration voltage of 300 kV anda magnification ratio of imaging of 450,000, and the imaging wasperformed so that entire region of the magnetic tape from the magneticlayer side surface to the back coating layer side surface in thethickness direction in one image. In the STEM image obtained asdescribed above, a linear line connecting both ends of a line segmentshowing the surface of the magnetic layer was determined as a referenceline showing the surface of the magnetic tape on the magnetic layerside. In a case where the STEM image was captured so that the magneticlayer side of the cross section observation sample was positioned on theupper side of the image and the back coating layer side was positionedon the lower side, for example, the linear line connecting both ends ofthe line segment described above is a linear line connecting anintersection between a left side of the image (shape is a rectangular orsquare shape) of the STEM image and the line segment, and anintersection between a right side of the STEM image and the line segmentto each other. In the same manner as described above, a reference lineshowing the interface between the magnetic layer and the non-magneticlayer, a reference line showing the interface between the non-magneticlayer and the non-magnetic support, a reference line showing theinterface between the non-magnetic support and the back coating layer,and a reference line showing the surface of the magnetic tape on theback coating layer side were determined.

The thickness of the magnetic layer was obtained as the shortestdistance from one position randomly selected on the reference lineshowing the surface of the magnetic tape on the magnetic layer side tothe reference line showing the interface between the magnetic layer andthe non-magnetic layer. In the same manner as described above, thethicknesses of the non-magnetic layer, the non-magnetic support, and theback coating layer were obtained.

(3) ΔN of Magnetic Layer

Hereinafter, M-2000U manufactured by J. A. Woollam Co., Inc. was used asthe ellipsometer. The creating and fitting of a double-layer model or asingle-layer model were performed with WVASE32 manufactured by J. A.Woollam Co., Inc. as the analysis software.

(i) Measurement Refractive Index of Non-Magnetic Support

A sample for measurement was cut out from each magnetic tape, the backcoating layer of the sample for measurement was wiped off and removedusing cloth permeated with methyl ethyl ketone to expose the surface ofthe non-magnetic support, and then, this surface is roughened with sandpaper so that reflected light of the exposed surface is not detected inthe measurement which will be performed after this with theellipsometer.

After that, by wiping off and removing the magnetic layer and thenon-magnetic layer of the sample for measurement using the clothpermeated with methyl ethyl ketone and bonding a surface of a siliconwafer and the roughened surface to each other using static electricity,the sample for measurement was disposed on the silicon wafer so that thesurface of the non-magnetic support exposed by removing the magneticlayer and the non-magnetic layer (hereinafter, referred to as the“surface of the non-magnetic support on the magnetic layer side”) facedupward.

The incidence ray was incident to the surface of the non-magneticsupport of the sample for measurement on the magnetic layer side on thesilicon wafer using the ellipsometer as described above, to measure Dand T. By using the obtained measurement values and the thickness of thenon-magnetic support obtained in the section (2), the refractive indexesof the non-magnetic support (the refractive index in a longitudinaldirection, the refractive index in a width direction, the refractiveindex in a thickness direction measured by incidence of incidence ray inthe longitudinal direction, and the refractive index in a thicknessdirection measured by incidence of incidence ray in the width direction)were obtained by the method described above.

(ii) Measurement of Refractive Index of Non-Magnetic Layer

A sample for measurement was cut out from each magnetic tape, the backcoating layer of the sample for measurement was wiped off and removedusing cloth permeated with methyl ethyl ketone to expose the surface ofthe non-magnetic support, and then, this surface is roughened with sandpaper so that reflected light of the exposed surface is not detected inthe measurement which will be performed after this with thespectroscopic ellipsometer.

After that, the surface of the magnetic layer of the sample formeasurement was wiped off using the cloth permeated with methyl ethylketone, the magnetic layer was removed to expose the surface of thenon-magnetic layer, and then, the sample for measurement was disposed onthe silicon wafer in the same manner as in the section (i).

The measurement regarding the surface of the non-magnetic layer of thesample for measurement on the silicon wafer was performed using theellipsometer, and the refractive indexes of the non-magnetic layer (therefractive index in a longitudinal direction, the refractive index in awidth direction, the refractive index in a thickness direction measuredby incidence of incidence ray in the longitudinal direction, and therefractive index in a thickness direction measured by incidence ofincidence ray in the width direction) were obtained by the methoddescribed above by spectral ellipsometry.

(iii) Measurement of Refractive Index of Magnetic Layer

A sample for measurement was cut out from each magnetic tape, the backcoating layer of the sample for measurement was wiped off and removedusing cloth permeated with methyl ethyl ketone to expose the surface ofthe non-magnetic support, and then, this surface is roughened with sandpaper so that reflected light of the exposed surface is not detected inthe measurement which will be performed after this with thespectroscopic ellipsometer.

After that, the sample for measurement was disposed on the siliconwafer, in the same manner as in the section (i).

The measurement regarding the surface of the magnetic layer of thesample for measurement on the silicon wafer was performed using theellipsometer, and the refractive indexes of the magnetic layer (therefractive index Nx in a longitudinal direction, the refractive index Nyin a width direction, the refractive index Nz₁ in a thickness directionmeasured by incidence of incidence ray in the longitudinal direction,and the refractive index Nz) in a thickness direction measured byincidence of incidence ray in the width direction) were obtained by themethod described above by spectral ellipsometry. Nxy and Nz wereobtained from the obtained values, and the absolute value ΔN of thedifference of these values was obtained. Regarding all of magnetic tapesof the examples and the comparative examples, the obtained Nxy was avalue greater than Nz (that is, Nxy>Nz).

(4) Vertical Squareness Ratio (SQ)

A vertical squareness ratio of the magnetic tape is a squareness ratiomeasured regarding the magnetic tape in a vertical direction. The“vertical direction” described regarding the squareness ratio is adirection orthogonal to the surface of the magnetic layer. Regardingeach magnetic tape which was manufactured, the vertical squareness ratiowas obtained by sweeping an external magnetic field in the magnetic tapeat a measurement temperature of 23° C.±1° C. using a vibrating samplemagnetometer (manufactured by Toei Industry Co., Ltd.) under conditionsof a maximum external magnetic field of 1194 kA/m (15 kOe) and a scanspeed of 4.8 kA/m/sec (60 Oe/sec). The measurement value is a valueafter diamagnetic field correction, and is obtained as a value obtainedby subtracting magnetization of a sample probe of the vibrating samplemagnetometer as background noise. In one aspect, the vertical squarenessratio of the magnetic tape is preferably 0.60 to 1.00 and morepreferably 0.65 to 1.00. In addition, in one aspect, the verticalsquareness ratio of the magnetic tape can be, for example, 0.90 orsmaller, 0.85 or smaller, or 0.80 or smaller, and can also be greaterthan these values.

[Missing Pulse Generation Frequency in Low Temperature and Low HumidityEnvironment]

The following measurement was performed in the low temperature and lowhumidity environment of a temperature of 13° C. and relative humidity of15%.

A magnetic tape cartridge accommodating each magnetic tape (magnetictape total length of 500 m) of the examples and the comparative exampleswas set in a drive of Linear Tape-Open Generation 6 (LTO-G6)manufactured by IBM, and the magnetic tape was subjected toreciprocating running 1,500 times at tension of 0.6 N and a runningspeed of 6 msec.

The magnetic tape cartridge after the running was set in a referencedrive (LTO-G6 drive manufactured by IBM), and the magnetic tape isallowed to run to perform the recording and reproducing. A reproducingsignal during the running was introduced to an external analog/digital(AD) conversion device. A signal having a reproducing signal amplitudewhich is decreased by 70% or greater than an average (average ofmeasured values at each track) was set as a missing pulse, a generationfrequency (number of times of the generation) thereof was divided by thetotal length of the magnetic tape to obtain a missing pulse generationfrequency (unit: times/m) per unit length (per 1 m) of the magnetictape. In a case where the missing pulse generation frequency is 5times/m or smaller, the magnetic tape can be determined as a magnetictape having high reliability in practice.

The results of the above evaluation are shown in Table 1 (Table 1-1 andTable 1-2).

TABLE 1-1 Comparative Comparative Comparative Comparative ComparativeExample 1 Example 2 Example 3 Example 4 Example 5 Preparation of Oxideabrasive product name Hit70 Hit80 Hit80 Hit100 Hit70 abrasive solution(manufactured by Sumitomo Chemical Co., Ltd.) Abrasive solution BETspecific surface 20 30 30 40 20 area (m²/g) Abrasive solution dispersionliquid 0 part 3.0 parts 3.0 parts 3.0 parts 3.0 parts(2,3-dihydroxynaphthalene) content Bead dispersion time 5 minutes 5minutes 60 minutes 180 minutes 60 minutes Centrifugation Rotation speedNone None 3500 rpm 3500 rpm 3500 rpm Centrifugation None None 3.8minutes 3.8 minutes 3.8 minutes time Filter hole diameter 0.5 μm 0.5 μm0.3 μm 0.3 μm 0.3 μm Preparation of Stirring time 5 minutes 30 minutes60 minutes 360 minutes 180 minutes magnetic layer Ultrasonic dispersionprocessing time 0.5 minutes 0.5 minutes 60 minutes 60 minutes 60 minutesforming Filter hole diameter 0.5 μm 0.5 μm 0.3 μm 0.3 μm 0.3 μmcomposition Filtration processing number Once Once Once Three timesTwice Ferromagnetic Average plate ratio 2.0 3.0 3.0 3.0 2.0 powderActivation volume 1600 nm³ 1600 nm³ 1600 nm³ 1600 nm³ 1600 nm³ Magneticliquid bead dispersion time 6 hours 50 hours 50 hours 50 hours 6 hoursMagnetic liquid dispersion bead diameter 1 mm 0.1 mm 0.1 mm 0.1 mm 1 mmMagnetic liquid 60 eq/ton 3300 eq/ton 330 eq/ton 330 eq/ton 60 eq/tonAmount of SO₃Na group in polyurethane resin Magnetic liquid 25.0 parts15.0 parts 15.0 parts 15.0 parts 25.0 parts Content of SO₃Na groupcontaining polyurethane resin Non-magnetic layer forming compositiondispersion time 3 hours 24 hours 24 hours 24 hours 3 hours Magneticlayer drying temperature 70° C. 50° C. 50° C. 50° C. 70° C. Calendertemperature 80° C. 100° C. 100° C. 100° C. 80° C. Formation andalignment of magnetic layer No alignment Homeotropic HomeotropicHomeotropic No alignment process alignment 0.5 T alignment 0.5 Talignment 0.5 T process Result Vertical squareness ratio (SQ) 0.50 0.660.66 0.66 0.50 ΔN 0.10 0.30 0.30 0.30 0.10 FIB abrasive diameter 0.20 μm0.16 μm 0.08 μm 0.03 μm 0.14 μm Missing pulse generation frequency 8 610 13 8 (time/m) Comparative Comparative Comparative Comparative Example6 Example 7 Example 8 Example 9 Preparation of Oxide abrasive productname Hit70 Hit70 Hit70 Hit70 abrasive solution (manufactured by SumitomoChemical Co., Ltd.) Abrasive solution BET specific surface 20 20 20 20area (m²/g) Abrasive solution dispersion liquid 3.0 parts 3.0 parts 3.0parts 3.0 parts (2,3-dihydroxynaphthalene) content Bead dispersion time60 minutes 60 minutes 60 minutes 60 minutes Centrifugation Rotationspeed 3500 rpm 3500 rpm 3500 rpm 3500 rpm Centrifugation 3.8 minutes 3.8minutes 3.8 minutes 3.8 minutes time Filter hole diameter 0.3 μm 0.3 μm0.3 μm 0.3 μm Preparation of Stirring time 180 minutes 180 minutes 180minutes 180 minutes magnetic layer Ultrasonic dispersion processing time60 minutes 60 minutes 60 minutes 60 minutes forming Filter hole diameter0.3 μm 0.3 μm 0.3 μm 0.3 μm composition Filtration processing numberTwice Twice Twice Twice Ferromagnetic Average plate ratio 2.0 3.0 6.03.0 powder Activation volume 1600 nm³ 1600 nm³ 1600 nm³ 1600 nm³Magnetic liquid bead dispersion time 6 hours 50 hours 96 hours 50 hoursMagnetic liquid dispersion bead diameter 1 mm 0.1 mm 0.1 mm 0.1 mmMagnetic liquid 60 eq/ton 330 eq/ton 330 eq/ton 330 eq/ton Amount ofSO₃Na group in polyurethane resin Magnetic liquid 25.0 parts 15.0 parts10.0 parts 15.0 parts Content of SO₃Na group containing polyurethaneresin Non-magnetic layer forming composition dispersion time 3 hours 24hours 48 hours 24 hours Magnetic layer drying temperature 70° C. 50° C.30° C. 50° C. Calender temperature 80° C. 100° C. 110° C. 100° C.Formation and alignment of magnetic layer Homeotropic No alignmentHomeotropic Second layer: alignment 0.5 T process alignment 0.5 T noalignment process/ftrst layer: homeotropic alignment 0.5 T ResultVertical squareness ratio (SQ) 0.55 0.53 0.80 0.60 ΔN 0.20 0.20 0.450.20 FIB abrasive diameter 0.14 μm 0.14 μm 0.14 μm 0.14 μm Missing pulsegeneration frequency 7 7 7 9 (time/m)

TABLE 1-2 Comparative Comparative Example 10 Example 11 Example 1Example 2 Preparation Oxide abrasive product name Hit70 Hit70 Hit70Hit70 of abrasive (manufactured by Sumitomo solution Chemical Co., Ltd.)Abrasive solution BET specific 20 20 20 20 surface area (m²/g) Abrasivesolution dispersion liquid 3.0 parts 3.0 parts 3.0 parts 3.0 parts(2,3-dihydroxynaphthalene) content Bead dispersion time 60 minutes 60minutes 60 minutes 60 minutes Centrifugation Rotation speed 3500 rpm3500 rpm 3500 rpm 5200 rpm Centrifugation time 3.8 minutes 3.8 minutes3.8 minutes 3.8 minutes Filter hole 0.3 μm 0.3 μm 0.3 μm 0.3 μm diameterPreparation Stirring time 180 minutes 180 minutes 180 minutes 180minutes of magnetic Ultrasonic dispersion processing time 60 minutes 60minutes 60 minutes 60 minutes layer forming Filter hole diameter 0.3 μm0.3 μm 0.3 μm 0.3 μm composition Filtration processing number TwiceTwice Twice Twice Ferromagnetic Average plate ratio 6.0 2.0 3.0 3.0powder Activation volume 1600 nm³ 1600 nm³ 1600 nm³ 1600 nm³ Magneticliquid bead dispersion time 96 hours 6 hours 50 hours 50 hours Magneticliquid dispersion bead diameter 0.1 mm 1 mm 0.1 mm 0.1 mm Magneticliquid 330 eq/ton 60 eq/ton 330 eq/ton 330 eq/ton Amount of SO₃Na groupin polyurethane resin Magnetic liquid 10.0 parts 25.0 parts 15.0 parts15.0 parts Content of SO₃Na group containing polyurethane resinNon-magnetic layer forming composition dispersion 48 hours 3 hours 24hours 24 hours time Magnetic layer drying temperature 30° C. 70° C. 50°C. 50° C. Calender temperature 110° C. 80° C. 100° C. 100° C. Formationand alignment of magnetic layer Second layer: no Second layer:Homeotropic Homeotropic alignment process/first homeotropic alignmentalignment 0.5 T alignment 0.5 T layer: homeolropic 0.5 T/first layer: noalignment 0.5 T alignment process Result Vertical squareness ratio (SQ)0.66 0.53 0.66 0.66 ΔN 0.20 0.20 0.30 0.30 FIB abrasive diameter 0.14 μm0.14 μm 0.14 μm 0.09 μm Missing pulse generation frequency 8 8 2 3(time/m) Example 3 Example 4 Example 5 Preparation Oxide abrasiveproduct name Hit80 Hit70 Hit70 of abrasive (manufactured by Sumitomosolution Chemical Co., Ltd.) Abrasive solution BET specific 30 20 20surface area (m²/g) Abrasive solution dispersion liquid 0 part 3.0 parts3.0 parts (2,3-dihydroxynaphthalene) content Bead dispersion time 60minutes 60 minutes 60 minutes Centrifugation Rotation speed 3500 rpm3500 rpm 3500 rpm Centrifugation time 3.8 minutes 3.8 minutes 3.8minutes Filter hole 0.3 μm 0.3 μm 0.3 μm diameter Preparation Stirringtime 60 minutes 180 minutes 180 minutes of magnetic Ultrasonicdispersion processing time 60 minutes 60 minutes 60 minutes layerforming Filter hole diameter 0.3 μm 0.3 μm 0.3 μm composition Filtrationprocessing number Twice Twice Twice Ferromagnetic Average plate ratio3.0 3.0 3.0 powder Activation volume 1600 nm³ 1600 nm³ 1600 nm³ Magneticliquid bead dispersion time 50 hours 50 hours 50 hours Magnetic liquiddispersion bead diameter 0.1 mm 0.1 mm 0.1 mm Magnetic liquid 330 eq/ton330 eq/ton 330 eq/ton Amount of SO₃Na group in polyurethane resinMagnetic liquid 15.0 parts 15.0 parts 15.0 parts Content of SO₃Na groupcontaining polyurethane resin Non-magnetic layer forming compositiondispersion 24 hours 24 hours 24 hours time Magnetic layer dryingtemperature 50° C. 50° C. 50° C. Calender temperature 100° C. 100° C.100° C. Formation and alignment of magnetic layer Homeotropic Secondlayer: Homeotropic alignment 0.5 T homeotropic alignment 0.2 T alignment0.5 T/first layer: no alignment process Result Vertical squareness ratio(SQ) 0.66 0.60 0.60 ΔN 0.30 0.35 0.25 FIB abrasive diameter 0.11 μm 0.14μm 0.14 μm Missing pulse generation frequency 3 3 4 (time/m)

From the results shown in Table 1, in the magnetic tapes of Examples 1to 5 in which ΔN and the FIB abrasive diameter of the magnetic layer arerespectively in the range described above, it is possible to confirmthat the missing pulse generation frequency in the low temperature andlow humidity environment is decreased, compared to the magnetic tapes ofComparative Examples 1 to 11.

In general, the squareness ratio is known as an index for a state of theferromagnetic powder present in the magnetic layer. However, as shown inTable 1, even in a case of the magnetic tapes having the same verticalsquareness ratios, ΔN are different from each other (for example,Examples 1 to 3 and Comparative Example 10). The inventors have thoughtthat this shows that ΔN is a value which is affected by other factors,in addition to the state of the ferromagnetic powder present in themagnetic layer.

One aspect of the invention is effective in a technical field of variousmagnetic recording media such as a magnetic tape for data storage.

What is claimed is:
 1. A magnetic tape comprising a magnetic layercontaining a ferromagnetic powder and a binding agent on a non-magneticsupport, wherein the magnetic layer contains an oxide abrasive, anaverage particle diameter of the oxide abrasive obtained from asecondary ion image acquired by irradiating a surface of the magneticlayer with a focused ion beam is greater than 0.08 μm and 0.14 μm orsmaller, and an absolute value ΔN of a difference between a refractiveindex Nxy measured with respect to an in-plane direction of the magneticlayer and a refractive index Nz measured with respect to a thicknessdirection of the magnetic layer is 0.25 to 0.40.
 2. The magnetic tapeaccording to claim 1, wherein the oxide abrasive is an alumina powder.3. The magnetic tape according to claim 1, wherein the ferromagneticpowder is a ferromagnetic hexagonal ferrite powder.
 4. The magnetic tapeaccording to claim 1, wherein the difference Nxy−Nz between therefractive index Nxy and the refractive index Nz is 0.25 to 0.40.
 5. Themagnetic tape according to claim 1, further comprising a non-magneticlayer containing a non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.
 6. The magnetic tapeaccording to claim 1, further comprising a back coating layer containinga non-magnetic powder and a binding agent on a surface of thenon-magnetic support opposite to a surface provided with the magneticlayer.
 7. A magnetic recording and reproducing device comprising: themagnetic tape according to claim 1; and a magnetic head.
 8. The magneticrecording and reproducing device according to claim 7, wherein the oxideabrasive is an alumina powder.
 9. The magnetic recording and reproducingdevice according to claim 7, wherein the ferromagnetic powder is aferromagnetic hexagonal ferrite powder.
 10. The magnetic recording andreproducing device according to claim 7, wherein the difference Nxy−Nzbetween the refractive index Nxy and the refractive index Nz is 0.25 to0.40.
 11. The magnetic recording and reproducing device according toclaim 7, wherein the magnetic tape further comprises a non-magneticlayer containing a non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.
 12. The magnetic recordingand reproducing device according to claim 7, wherein the magnetic tapefurther comprises a back coating layer containing a non-magnetic powderand a binding agent on a surface of the non-magnetic support opposite toa surface provided with the magnetic layer.